Phototube cathode formation



May 4, 1948. I w $ALINGER 2,440,917

' PHOTOTUBE CATHODE FORMATION Filed March 8, 1946 PUMP IR FILTER SELECTIVE FILTER I INVENTOR HANS W. G. SALINGER ATTORNEY Patented May 4,1948

PHOTOTUBE CATHODE FORMATION Hans W. G. Salinger, Fort Wayne, Ind., assignor, by mcsne assignments, to Farnsworth Research ilorporation,

10 Glaims.

This invention relates to the formation of photoelectric cathodes and particularly to the formation of such cathodes for maximumresponse to particular regions of the radiant energy spectrum.

Ordinary uses for phototubes require them to function in response to light which generally consists of a number of components of difierent wave lengths usually within the visible portion of the spectrum. The cathodes of such phototubes may be formed by conventional methods to give a satisfactory response to radiant energy of such a character. It is well known, however, that any phototube characteristically does not have the property of producing equal responses to It frequently is desirable to produce a phototube having a cathode which will give a sible to produce two tubes having substantially the infra-red radiation.

of any particular wave lengths. The two responses may be considerably difierent.

a maximum response ergy of a predetermined wave length.

Another ode formation process.

A further object of the invention is to provide a method a corporation of Indiana Application March 8, 1946, Serial No.

It, therefore,

tensity of the projected energy is modified intermittently. At the same time, sensitizing material is deposited on the cathode. The photoelectric ou put of the phototube representative of the radiation of predetermined which is projected onto the cathode of the invention, there eluding an episcotister for projecting intermittently upon the cathode of a phototube, radiant energy of the desired wave length. Means also point of optimum sensitization of the cathode by appended claims.

The single fi ure is a diagrammatic representation of apparatus for carrying out the novel method .of cathode formation in accordance with the invention.

Having reference now shown a phototube of the accompanying drawing 01' one form the cathode.

v the infra-red light.

the cathode and anode. For example, in addition to the cathode and anode, there may be mounted in the envelope one or more electron multiplying electrodes (not shown) having secondary electron emlssive properties and any desired forms. The envelope I is provided with a tubulation 4 which is connected to an exhaust pump 5. This arrangement enables the evacuating of the main tube envelope which is necessary during the formation or the sensitized cathode surface and also for the completed tube. The main body of the envelope i preferably is enclosed within an oven (not shown) for heating to a temperature such as 200 degrees centlgrade in accordance with well known practice.

Substantially any known process may be employed for depositing sensitizing material upon the cathode 2. As illustrated herein, the tubulation 4 contains a number of pellets such as 6 which-may be of a caesium salt such as caesium chromate to which a suitable reducing agent is added whereby, upon the application of heat, metallic caesium is produced for deposition upon In such a case, the portion of the tubulation 4 which contains the caesium chromate pellets is surroundedby a heating coil 1. This coil is connected through a switch 8 to a source of energy such as that represented by the battery 9.

It is contemplated that, in carrying out the process in accordance with this invention, other means for sensitizing the cathode may be employed. For example, the sensitizing material such as metallic caesium may be introduced into the phototube in small glass tubes which may be heated by means such as a to evaporate the caesium, in which state it is easily deposited on the cathode tube. Also, in some cases where increased sensitivity is desired, silver may be deposited over the caesium. All such processes for sensitizing the cathode are contemplated to come within the scope of this invention.

In accordance with one of the novel features of the present invention, there also are provided facilities for projecting radiant energy by a predetermined wave cathode 2 during the sensitization of this surface. Preferably this radiant energy is modified intermittently in intensity for a purpose to be described. One means for projecting such radiant energy onto the cathode surface comprises a source of light energy H which includes components of several different wave lengths. It will be assumed for the purpose of the present description that it is desired to produce a phototube having a cathode which has a maximum response to infra-red light. In such a case, the source II will be capable of producing radiant energy including energy at the wave length of The light emanating from the source II is projected by means of a lens system l2 intoa beam whereby an image of the source is formed in a plane located between, the source and the cathode 2. There is mounted in the image plane of. the light source Ii, apparatus for modifying the intensity of the beam. This apparatus comprises essentially an episcotister i3 such as a perforated disc which may be rotated by any suitable means (not shown). In the illustrated form of this apparatus, the disc is provided adjacent its periphery with a plurality of periorations such as M. In such a case, the perio Hnn are arranged so that they move in of transmitting only gas torch so as length upon the surface of the m e plane oi the light source H sively coincide with the relatively small image point or the light source. By such means, the light beam is alternately transmitted through the perforations and interrupted by the opaque portions of the disc between the perforations.

It is contemplated to be within the scope of the invention that the episcotister employed may have any one of several other different forms. For example, instead of the perforations such as H, the disc l3 may be provided with a plurality of serrations made in the peripheral edge thereof. Furthermore, the perforations, serrations. or whatever other form of structure is used may be entirely transparent whereby to transmit the light beam in full intensity or they may be semi-transparent or translucent, whereby to transmit the light beam in less than its maximum intensity. The only essential requirement of an episcotister for use in practicing this invention is that it be capable of intermittently modifying the intensity of the light beam.

The apparatus further includes a condensing lens system represented at is which is located between the episcotister i3 and the cathode 2. The function of the condensing lens is conventional; that is, to concentrate a relatively large portion of the light beam substantially uniformly over the surface of the cathode 2.

In the illustrated form of the apparatus embodying the invention, a filter element i6 capable the radiant energy of the predetermined wave length such as infra-red light, in the assumed case, is located between the condensing lens i5 and the cathode 2. In view of the foregoing description of the episcotister l3, it will be apparent that it is not necessary in all cases to provide a separate filter element i6 located as illustrated herein. It may be desirable to incorporate the filter element in the episcotister. One way of accomplishing this is to provide each one of the disc apertures such as H with individual filter elements having the desired wave length transmission properties. Other alternative arrangements of apparatus components 'will be suggested tov those skilled in the art. It is contemplated, however, that all such apparatus organizations, whereby radiant energy of the desired; wave length is projected upon the surface of the cathode being sensitized and is intermittently-modified in intensity, come within the scope of the present invention. Where the tube envelope I is enclosed within an oven, a suitably located window is provided in the oven to enable the described projection of radiant energy upon the cathode 2.

The output circuit of the phototube includes a load impedance device such as a resistor l! which together with a source of energy such as battery 18 is connected between the cathode 2 and the anode 3 of the phototube. The output circuit also includes a selective filter network l9 and an amplifier 2|. The output circuit of the phototube may be entirely conventional. While the selective filter I9 is shown as a separate component. it may be incorporated in a conventional manner either with the load circuit resistor II or with the amplifier IS. The essential property of the filter network is to produce signal effects which are representative only of the intermitand succes- 'tently modified radiant energy of the desired wave length which of the phototuloe. the amplifier 2| is the II representative of the radiant energy of the deoscilloscope 22. Specifically thereof, that the intensity select the signal efi'ects representative of the radiant energy of the desired wave length from the signal effects representative of radiant energy of other wave lengths to which the oathode may be exposed.

The output circuit of the phototube, which may include the described filter and amplifier, is coupled to an indicating device which in the present instance is illustrated as a cathode ray in accordance with the arrangement shown, the output circuit of the amplifier 2| maybe coupled to the deflection elements of the oscilloscope by means of which the amplitude of the cathode ray deflection in one direction is controlled to produce traces on the oscilloscope screen representative of the phototube response to the radiant energy of the desired wave length.

In operating the described apparatus for carry lng out the novel method of the invention, the light source H is energized and the episcotister i3 is rotated at a predetermined speed. It will be assumed for the purpose of the present description that the speed of rotation of the episco tister is such, in relation to the apertures Id of the light beam is modified approximately at the rate of one thousand times per second. In practicing the instant invention, the rate at which the intensityof the light beam is modified is more or less immaterial. Generally, however, unidirectional energy for use in measuring the response of the cathode is produced by a rectifying alternating current derived from a commercial source. Such alternating current energy usually is supplied at a frequency of 60 cycles per/second. As is'well known,

' regardless of the eificiency of filters used in con- Junction with such rectifiers, it is not sufiiciently high to eliminate all traces of the 60 cycle component from the signal effects produced in the output circuit of the phototube. Consequently; it is desirable to modify the intensity of the light beam at some frequency other than 60 cycles and all multiples thereof. It has been found that a rate of one thousand times per second for this purpose gives satisfactory results. Not only does it afford a means for distinguishing between the ing the desired output from spurious signal effccts whichmay be produced by other means such as thermionic electron emission by the cathode. In the assumed case, the infra-red light which is projected onto the cathode of the tube is modified in intensity one thousand times is condensed upon which it is desired scribed, phototubes in the load resistor IT. 76

,of the desired used advantageously where it In view of the wave length is modified before projection upon the cathode, in practicing this invention it is not necessary to use any special precautions for shielding the cathode from extraneous light the modified light energy The cathode sensitization thus is terminated. In accordance with well known practice, in addition to de-energizing the heating coilv I. there may be included additional facilities for' simultaneously cooling that portion of the tubulation 4 which contains the caesium chromate pellets 6. By such means the sensitization process may be terminated more quickly.

It maybe seen that, by the employment of the method of phototube cathode sensitization demay be produced so that all have substantially identical operating characteristics. Tube spoilage thus will be reduced materially over that of the present where cut and try methods of cathode formation are employed.

have identically the same response characteristics, even though in some cases individual tubes may be capable upon further sensitization of greater responses. It merely is necessary to fix quantitatively a desired output level for a number of 'phototubes and then to carry the cathode sensitization process only to the point where the desired output is produced. Furthermore; as previously suggested, the present process may be is undesirableor inconvenient to take precautions against exposing the cathode to extraneous light during the pathode sensitization period.

It is contemplated to be within present invention that it may forming the cathodes of any electron discharge tubes which are required to have a predetermined response to radiant energy of substantially any predetermined wave length. Moreover-cathodes ofelectron discharge tubes may be formed by the the scope of the 6 fact that only the radiant energy of other wave lengths. Conse- 1 quently, the photoelectric response by the cathode be practiced in 1 accordingly is not intended to presehtxprocess and apparatus when such cathodes are required to have a maximum response to radiant energy covering in some cases a relatively wide range of wave lengths. Obviously, in such cases, it may be possible to eliminate the light filter and to merely modify the intensity of the radiant energy projected ontothecathode.

Furthermore, it may be desired to form the cathode of an electron discharge device which will have optimum response to more than one radiant energy component. For example, the present invention may be used advantageously to form a cathode which is to be responsive to radiant energy components of two different predetermined wavelengths. In such a case, two separate projecting systems both similar in character to the one disclosed herein may be employed. Each projecting system will be utilized to subject the cathode to one particular component of radiant energy having a predetermined wave length. In the case where it is desired to 'difierentiate between the cathode response to, the two different components, suitable apparatus may be employed to modify the intensity of the respective beams of radiant energy at two different rates. Selective filter networks corresponding to the rates of intensity modification will be in luded in the output circuit. The cathode sensitizationmay be effected in such a case so as to produce a desired balance in the response by the cathode to the difieernt components of radiant energy. In view of the contemplated scope of the invention, it is to be understood that the references to the formation of a cathode for a phototube made in this specification and in the following claims merely is for the purpose disclosing a particular embodiment of the invention and such language be construed as at all limiting in this respect.

Furthermore, it will be obvious to those skilled in the art that the invention may be practiced by the use of various forms of apparatus other than those disclosed specifically herein for illustrative purposes. For example, in practicing this invention, it is essential only to expose the cathode which is to be formed to radiant energy or the predetermined wave length, the intensity of which energy is modified intermittently so as to be distinguishable from radiant energy of other wave lengths to which the cathode may be exposed at the same time. For example, instead of using a source of white light such as the disclosed source ll, there may be employed a source of radiant energy having only one component which is that of the desired wave length. Also, as previously suggested, means other than those disclosed for modifying the intensity or the radiant energy may be used without departing from the invention. 7

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for forming a cathode of an electron discharge tube for predetermined response to radiant energy of a predetermined wave length comprising, a cathode to be sensitized, means for directing a beam of radiant energy of said predetermined wave length onto said cathode, means the response by 8 for periodically modifying the intensity of said beam, means coupled to said cathode to register said cathode to said periodically modified radiant energy oi! predetermined wave length, and means for concurrently depositing sensitizing material upon said cathode.

2. Apparatus for forming a cathode of an electron discharge tube for predetermined response to radiant energy of a predetermined wave length comprising, a tube envelope, a cathode to be sensitized in said envelope, means for developing a beam of radiant energy of said predetermined wave length, means for directing said beam of said radiant energy onto said cathode, means located in the path of said beam for intermittently modifying the intensity of said beam, means coupled to said cathode for developing a voltage representative of the response by said cathode to the radiant energy of predetermined wave length received thereby, means coupled to said voltage developing means to register the response by said cathode to said radiant energy of predetermined wave length, and means for depositing sensitizing material upon said cathode until the predetermined response by said cathode to said radiant energy of predetermined wave length is registered upon said indicatin device said by said registering means.

3. Apparatus for forming a. cathode of an electron discharge tube for maximum response to radiant energy or a predetermined wave length comprising, a tube envelope, a cathode to be sensitized in said envelope, a source of radiant energy of saidpredetermined wave length, means for directing a beam of said radiant energy onto said cathode. means including an episcotister located in the path of said beam for intermittently modifying the intensity of said beam, means including an output circuit connected to said cathode ior developing a voltage representative of the response by said cathode to the radiant energy of predetermined wave length received thereby, means including an indicator coupled to said output circuit to register the response by said cathode to said radiant energy of predetermined wave length, and means for depositing sensitizing material upon said cathode until the maximum indication by said indicator is registered.

4. Apparatus for forming a photoelectric cathode of a phototube for maximum response to in- Ira-red radiation comprisinga tube envelope, a cathode to be photosensitized in said envelope, a source of light, means including an optical syslight into a beam and for projecting said beam onto said cathode, an episcotister located in the path or said beam for periodically interrupting said light beam, an infrared filter element located in front of said cathode, an output circuit for said phototube connected to said cathode for developing a signal voltage representative of the photoelectric response by said cathode to the interrupted infra-red radiations received thereby, means connected to said phototube output circuit for developing amplified signal voltages representative or the infra-red output of said phototube, an indicating device, means including a filter network for impressing amplified signal voltages representative of the interrupted infrared radiation projected onto said cathode, and means for evaporatingphoto-sensitizing material upon said cathode until the maximum indication by said indicating device is produced.

5. Apparatus foriorming a photoelectric cathode or a phototube for maximum response to infra-red radiation comprising. a tube envelope,

a cathode to be photosensitized and an anode in said envelope, a tubulation extending from said envelope and including caesium sensitizing material, means for connecting said tubulation to an exhausting pump for evacuating said envelope, a source of light, an optical system for forming said light into a beam, a rotating disc having a plurality of apertures located in the path of said light beam for interrupting said light beam at a predetermined frequency, an infra-red filter element located in said beam in front of said cathode, means for projecting said interrupted light beam through said filter and onto said cathode, an output circuit for said phototube connected between said cathode and said anode for developing a voltage representative of the total photoelectric response by said cathode to the light radiations received thereby, a filter network coupled to said phototube output circuit for selecting only the si nal voltages representative of the interrupted infra-red radiation projected onto said cathode, an amplifier having an input circuit connected to said filter network and an output circuit for developing amplified signal voltages representative of the output of said phototube, an indicating device connected to said amplifier output circuit for control thereby to produce visible indications of the infra-red response of said phototube cathode, and means for heating that portion of said tubulation including the caesium sensitizing. material for effecting evaporation of said caesium until the maximum indication of infra-red response is produced on said indicating device.

6. Apparatus for forming a photoelectric cathode of a phototube for maximum response to infra-red radiation comprising, a tube envelope, a cathode to be photosensitized and an anode in said envelope, a tubulation extending from said envelope and including a plurality of caesium means for connecting said tubulation to an exhausting pump for evacuating said envelope, a source of white light, an optical syssaid light into a beam whereby to create an image of said light source in a predea rotating disc having a plurality of apertures located in said predetermined plane for interrupting said light beam at a predeterfrequency, an infra-red filter elephotoelectric response by said cathode to the light radiations reproduced on said oscilloscope screen.

7. The method of forming a-photoelectric cathode for predetermined response to. radiant energy of a predetermined wave length comprising termined indication is observed lo the steps of, projecting onto the cathode to be formed a beam of radiant energy of said predetermined wave length having an intensity which is periodically modified, depositing sensitizing material on said cathode, amplifying the photoelectric output of wave length, amplified photoelectric output.

resting said deposition of caesium when a prededevice.

mined frequency, filtering said interrupted beam to obtain only the infra-red component thereof, projecting said infra-red interrupted radiant enfrom its response to said interrupted infra-red radiant energy only the output of said cathode de-,

scope.

HANS W. G. SALI'NGER.

REFERENCES CITED The following file of this patent:

UNITED STATES PATENTS Number Name Date 2,008,874 Olpin July 23, 1935 2,029,040 Stilwell Jan. 28, 1986 2,128,582 Gardner Aug. 30, 1938 references are of record in the 

